Known methods for the production of complex shaped controlled porosity adsorbent material are discussed in WO 2004/087612 (Blackburn and Tennison, the disclosure of which is incorporated herein by reference). The inventors explain that there are very few viable routes for the production of complex shaped controlled porosity adsorbent materials. For instance, they explain that activated carbon is traditionally produced by taking a char made by pyrolysing an organic precursor or coal, grinding the char to a fine powder, mixing this with a binder, typically pitch, and extruding or pressing to give a “green” body. The green body is then further fired to pyrolyse the binder and this is then typically further activated in steam, air, carbon dioxide or mixtures of these gases to give the high surface activated carbon product. The drawback to this route is that the binder, which is usually a thermoplastic material, goes through a melting transition prior to pyrolytic decomposition. At this point the material is weak and unable to support a complex form. This, combined with the problems of activating the fired body, typically limits the size and shape of the products to simple extrudates.
An alternative route is to take an activated carbon powder and form this directly into the final shape. In this instance a range of polymeric binders have been used that remain in the final product. The main drawback to this route is that high levels of binders are required and these then tend to both fill the pores of the activated carbon powder and encapsulate the powder, leading to a marked reduction in adsorption capacity and a deterioration in the adsorption kinetics. The presence of the polymeric phase also degrades the physical and chemical stability of the formed material, severely limiting the range of applicability.
A further alternative is to take a formed ceramic material, such as a multichannel monolith, and to coat this with a carbon forming precursor such as a phenolic resin. It can then be fired and activated to produce a ceramic-carbon composite. The main limitations of this route are the cost associated with the ceramic substrate and the relatively low volume loading of carbon. At high degrees of activation it is possible to produce a mesoporous carbon although the carbon volumetric loading and the mechanical stability of the carbon is further reduced.
The applicants have previously developed methods for making carbonised and optionally activated monoliths from phenolic resin precursors. Monolithic porous carbon can be made by partially curing a phenolic resin to a solid, comminuting the partially cured resin, extruding the comminuted resin, sintering the extruded resin so as to produce a form-stable sintered product and carbonising and activating the form-stable sintered product. EP 0 254 551 (Satchell et al., the contents of which are incorporated herein by reference) gives details of methods of forming porous resin structures suitable for conversion to porous carbon structures. WO 02/072240 (Place et al . . . the disclosure of which is incorporated herein by reference) gives further details of producing monolithic structures using sintered resin structures of EP 0 254 551.
In this process for producing carbon monoliths, the resin cure is controlled so that it is sufficient to prevent the resin melting during subsequent carbonisation but low enough that the resin particles produced during the milling step can sinter during subsequent processing. The amount of crosslinking agent and the temperature and duration of the partial curing step are selected as to give a degree of cure sufficient to give a sinterable product, and such that a sample of the partially cured solid when ground to produce particles in the size range 106-250 μm and tableted in a tableting machine gives a pellet with a crush strength which is not less than 1 N/mm. Preferably the pellet after carbonisation has a crush strength of not less than 8 N/mm.
The comminuted resin particles may have a particle size of 1-250 μm, in embodiments 10-70 μm. In further embodiments the resin powder size is 20-50 μm which provides for inter-particle channels of size of 4-10 μm with an inter-particle channel volume of 30-40%. The size of the particles is selected to provide a balance between diffusivity through the inter-particle voids and within the particles.
As disclosed in U.S. Pat. No. 6,964,695 (Place et al., Carbon Technologies) the milled powder can then be extruded to produce polymeric monolithic structures with a wide range of cell structures, limited only by the ability to produce the required extrusion die, or other forms such as rods, tubes, trilobes etc. Suitable dies are well known to those skilled in the art. At this stage the monolith has a bimodal structure—the visible channel structure with either the central channel in a simple tube or the open cells in a square channel monolith of typically 100-1000 μm cell dimension and cell walls with thickness typically 100-1000 μm and the inter-particle void structure within the walls generated by the sintered resin particles.
Carbonisation takes place preferably by heating above 600° C. for the requisite time e.g. 1 to 48 hours and takes place under an inert atmosphere or vacuum to prevent oxidation of the carbon. On carbonisation the material loses about 50% weight and shrinks by about 50% volume but, provided the resin cure stage was correctly carried out, this shrinkage is accommodated with little or no distortion of the monolith leading to a physical structure identical to that of the resin precursor but with dimensions reduced by ˜30%. The inter-particle void size is also reduced by ˜30% although the void volume (ml/ml) remains essentially unaltered. During carbonisation the microstructure of the porous carbon develops, particularly at temperatures above 600° C. After carbonisation there may be partial blocking of the microstructure by the decomposition products from the carbonisation process. These blockages may be removed by activation to provide rapid access to the internal structure of the carbon that may be desirable for the operation of the monoliths as adsorption devices.
This production route is limited to the use of Novolac resins and this in turn limits the pore structure that can be produced to the approximately 1 nm pores that are characteristic of all novolac derived carbons and larger macropores, typically greater than 1 μm, that are produced by the voids between the sintered particles. It is not possible by this route to produce products with pores in the large meso-small micro range of sizes.
In US 2013/0072845 (Tennison et al.) a method is described for extending the porosity of the above structures to include meso and or small macro pores in addition to the micropores that derive from the novolac resin. In this invention solid particles of a first phenolic resin which is partially cured so that the particles are sinterable but do not melt on carbonisation are mixed with particles of a second phenolic resin that has a greater degree of cure than said first phenolic resin and has a mesoporous and/or macroporous microstructure generated by solvent pore forming that is preserved on carbonisation; forming the mixture into a dough; extruding the dough to form a shaped product and stabilizing its shape by sintering.
In the above method, the dough may be extruded to form a shaped body having walls defining a multiplicity of internal channels for fluid flow, the channels being directed along the extrusion direction e.g. as discussed in relation to FIG. 1. There may further be carried out the step of carbonising the resin, and optionally activating the carbonised resin.
In this production route the secondary, highly cured, meso/macro porous resin component is not strongly bound into the structure due to its high degree of cure but rather is trapped in a cage formed by the sinterable resin component. This approach limits the amount of the second material than can be incorporated due to the requirement to form the cage structure. This leads to a reduction in strength when compared to the materials produced entirely from the sinterable resin particles. The extent of the larger pore structure is also limited by dilution of the matrix with the first resin component. This production route can also be used with second components other than phenolic resin such as activated carbons but in this instance differential shrinkage between the particles comprising the cage and the second component during the pyrolysis process leads to stress cracking and a further reduction in mechanical strength. Thus whilst it is possible to produce complex shapes the reduced meso/macro pore capacity and strength limits there used in demanding applications such as blood filtration.
The production of the large meso/small micro pore carbons is described in U.S. Pat. No. 8,383,703 (Tennison et al., 2103) which is incorporated herein by reference. The preferred route for producing these materials is through the use of pore formers where ethylene glycol is the preferred component although other solvents may also be used. These meso/macro porous resins can be produced either as beads or as powders. In the routes described in US 2008/025907 (Tennison et al.) the precursors, typically comprising the novolac resin and the curing agent (typically hexamethylenetetramine (HTMA)) are dissolved in the pore forming solvent (typically ethylene glycol) in the ratios necessary to generate the required pore structure and degree of cure. The mixture can either be cured by dispersing in hot oil to form beads or placed in trays and cured in an oven. In the latter case the block of cured resin is subsequently processed by milling to give either the finished powder or a precursor for extrusion. The limitation of this route is the strength, attrition resistance and control of porosity in monolith materials produced by this route is insufficient to allow these materials to be used in demanding applications such as blood filtration.
There is therefore a requirement for a production route that permits the production of complex shaped nanoporous carbons with multimodal pore structures with sufficient strength and attrition resistance to allow their use in applications such as haemofiltration